Plastic has been used as a material of construction for various articles. Perhaps one of the more recent applications for plastic is its use as a material of construction for furniture and the like. It has been found that plastic can be molded to have a grained surface which is imperceptively different from that of wood. Generally for these applications the plastic is foamed rather than solid. A method of injection molding foamed plastics to produce fabricated articles is disclosed in U.S. Pat. No. 3,268,636, entitled "Method and Apparatus for Injection Molding Foamed Plastic Articles", issued Aug. 23, 1966. That patent teaches a process for molding foamed thermoplastic articles which includes the steps of mixing a blowing agent and a thermoplastic material, maintaining at a temperature at least equal to the melting temperature of the thermoplastic material, and a pressure and temperature above the pressure and temperature at which the mixture foams. A mold which is maintained at a pressure below the pressure at which the mixture foams is rapidly filled with the mixture which then foams and expands and fills out the mold.
Two problems have arisen in using foamed plastics as materials of construction for various molded articles. The first of these problems is that when plastics are molded, especially foamed plastics, there results a swirl-pattern in the surface of the finished article. Frequently when the finished article is furniture the swirl-pattern is noticeable in spite of the fact that the article is molded to have a grained surface.
The second problem is present when foamed articles are being produced by injection molding. More specifically, when a heat plastified plastic material which is being foamed by an active foaming agent is injected into a mold, a surface skin is formed by virtue of the fact that the heat plastified material contacts the mold walls which are below the heat plastifying temperature of the thermoplastic resin. However, after the mold has filled and the surface skin has formed, the foaming agent in the thicker uncooled section continues to produce gas. Concomitant with this continued gas production, small cells already formed migrate toward each other and combine to form larger cells. The end result, quite frequently, is that large voids are formed internally in the foamed article. These large voids are undesirable for two reasons. The first reason is that they frequently occur too close to the skin surface of the foamed article, and thus the skin surface is easily punctured. The second reason is that the voids substantially reduce the strength of the foamed article.
The invention of this disclosure allows the production of polymeric resinous articles, especially foamed polymeric thermoplastic articles which are substantially free of swirl-patterns in their surface as well as substantially free of large voids within the articles themselves.